Subterranean drilling typically involves rotating a drill bit on a downhole motor at the remote end of a string of drill pipe. The rotating bit works its way through underground formations opening a path for the drill pipe that follows. Drilling fluid forced through the drill pipe may rotate the motor and bit. The assembly may be directed or steered from a vertical drill path in any number of directions. Steering allows the operator to guide the wellbore to desired underground locations. For example, to recover an underground hydrocarbon deposit, the operator may first drill a vertical well to a point above the reservoir. Then the operator may steer the wellbore to drill a deflected, or directional, well that optimally penetrates the deposit. The well may pass horizontally through the deposit. The greater the horizontal component of a well or bore, the greater the friction between the bore and the drill string. This friction slows drilling by reducing the force pushing the bit into new formations.
Directional drilling, or steering, is typically accomplished by orienting a bent segment of the downhole motor driving the bit. Rotating the drill string changes the orientation of the bent segment and the "toolface", and thus the direction the bit will advance. To effectively steer the assembly, the operator must first determine the current toolface orientation. The operator may measure the toolface orientation with what is commonly known as "measurement while drilling" or "MWD" technology. If the drilling direction needs adjustment, the operator must rotate the drill string to change the orientation of toolface.
If no friction acts on the drill string or if the drill string is very short, simply rotating the drill string will correspondingly rotate the segment of pipe connected to the bit. However, during directional drilling, the drilling operator deflects the well or bore over hundreds of feet so that the bend in the drill string is not sudden. Thus directional drilling is often performed at the end of a drill string that is several thousand feet long. Also, directional drilling increases the horizontal component of a well and thus increases the friction between the drill string and the well. The drill string is elastic and stores torsional tension like a spring. The drill string may require several rotations at the surface to overcome the friction between the surface and the bit. Thus, the operator may rotate the drill string several revolutions at the surface without moving the toolface.
Typical drilling drives, such as top drives and independently driven rotary tables, prevent drill string rotation with a brake. To adjust the orientation of the toolface, the operator must release the brake and quickly supply sufficient power to the motor to overcome the torsional tension stored in the drill string and to advance the drill string the appropriate amount at surface to reorient the toolface at the end of the drill string. If the brake is released and insufficient power is supplied to the motor, the drill string will backlash. If too much power is supplied to the motor, the motor will quickly rotate the toolface past its desired orientation. If the initial brake release and motor power-up are successful, the operator must then stop the motor with the brake once the operator thinks the drill string has rotated sufficiently to properly reorient the toolface. If the operator's guess is too high, the motor will rotate the toolface past the desired orientation. If the operator's guess is too small, the motor may rotate the drill string at the surface but the toolface will not rotate sufficiently to be properly oriented.